1. Field of the Invention
The present invention relates generally to an optometer for an eyesight test, or for measuring the range of vision of an eye under examination, such that a series of optotypes or indicia are presented and viewed through a view finder or observation window formed through the body of the optometer, so that an optometrist can check if a person under examination can correctly recognize the optotypes through the eye. In particular, this invention is concerned with such an optometer which is compact and simple in structure and is capable of presenting a significantly increased number of optotypes to be seen through the window.
1. Discussion of Related Art
A conventional optometer or eyesight testing instrument used for examining the visual acuity of a human eye has a rotary optotype disk which has a plurality of optotypes such as Landolt's Rings, which are arranged in the circumferential direction of the disk. The optotype disk is rotatably supported in the body of the optometer having a window through which the optotypes are viewed by an eye, such that the optotypes are selectively brought into alignment with the window by rotation of the rotary optotype disk.
For enhancing the accuracy and reliability of the an eyesight test, it is desirable to use as many optotypes as possible. However, increasing the number of the optotypes arranged in the circumferential direction of the rotary disk undesirably results in an increased size of the optotype disk itself and an accordingly increased size of the optometer apparatus.
In the optometer constructed as described above, therefore, various devices and proposals have been made for presenting as many optotypes as possible before a subject person through the observation window, without increasing the size of the apparatus. For example, a relatively large number of optotypes or Landolt's Rings 11 having different sizes and orientations are provided on a rotary optotype disk 28, along a plurality of concentric circles about the center of the disk 28, as shown in FIG. 5. This optotype disk 28 is used with a shutter disk (not shown) which is rotatably disposed between the optotype disk 28 and a wall of the optometer body having the observation window. The shutter disk is dimensioned so as to cover substantially the entire surface area of the optotype disk 28, and has a plurality of openings which correspond to the respective concentric circular arrays of the optotypes 11 on the disk 28. The disk 28 is rotated while a selected one of the openings of the shutter disk is kept in alignment with the window of the optometer, so that the optotypes 11 in the circular array corresponding to the selected opening can be viewed one after another by the eye under examination through the window. With the optometer thus constructed, a relatively large number of optotypes 11 can be provided over the entire area of the front surface of the rotary optotype disk 28, and thus the size of the disk 28 can be advantageously reduced, with a result of reduction in the size of the optometer apparatus as a whole, as compared with a conventional counterpart wherein optotypes are arranged in a single row in the circumferential direction of a rotary optotype disk.
Another example of the optometer is disclosed in JP-B-53-3598, which includes a plurality of optotype disks 52 that are provided with respective optotypes 11 having different sizes, as shown in FIG. 6. These disks 52 are rotatably mounted in the peripheral portion of the rotary disk 28, such that the centers of the disks 52 lie on a circle that is concentric with the rotary disk 28. The rotary disk 28 and the optotype disks 52 are rotated by respective motors 54, 56, through respective chains 60, 62, under the control of limit switches 58, so that a desired one of the optotypes 11 which is oriented as desired is presented through an observation window (not shown). This arrangement eliminates a need to provide the rotary disk 28 with a plurality of optotypes 11 having the same size, and requires a relatively reduced number of optotypes 11, leading to reduction in the sizes of the rotary disk 28 and the optometer itself.
However, the known optometers as described above have various problems. Described more specifically, in the first example as shown in FIG. 5, which uses the rotary optometer disk 28 provided with a large number of optotypes 11 having different sizes and different orientations, no means are provided to reduce the number of optotypes 11 on the disk 28, though the arrangement of the optotypes 11 gives some advantage over the prior apparatus. Therefore, in order to further enhance the accuracy of an eyesight test or vision analysis, for example, a further increased number of optotypes 11 are necessary, thus inevitably increasing the size of the optotype disk 28, and eventually making the optometer apparatus undesirably large-sized. In the second example as shown in FIG. 6, the rotary disk 28 is provided with the optotype disks 52 the number of which is equal to the number of the optotypes 11 required, and the rotary disk 28 and the optotype disks 52 are rotated by the respective motors 54, 56. This inevitably makes the optometer structure undesirably complicated.